Milk fat of humans and other species includes significant quantities of triglycerides containing medium chain fatty acids, is readily digestible and provides the neonate with a major energy source at a time when its lipid-digesting apparatus has not fully matured. The ingestion of milk fat by neonates induces a mild ketosis; these ketones are apparently important for growth and development of the brain. Our program is directed toward elucidating the mechanisms which regulate the production of this important nutrient by mammay gland. We have shown that in rats, mice and humans, the biosyntesis of medium chain fatty acids requires two key enzymes, fatty acid synthetase and thioesterase II, a mammary-specific, chain-terminating enzyme. Employing both enzymological and immunohistochemical techniques with collagen gel culture systems that simulate the mammogenic and lactogenic phases of development occurring in rats, mice and humans, we will: (i) determine whether the program of differentiation of the mammary lipogenic system of the human resembles more closely that of the rat, in which fatty acid synthetase and thioesterase II are expressed asynchronously, or the mouse, in which they are expressed synchronously, (ii) determine the requirement for polypeptide and steriod hormones in regulating induction and maintenance of the two enzymes, (iii) determine the role of protein synthesis and degradatio in the exercising of hormonal regulation of the lipogenic system. Fully differentated mammary epithelial cells isolated from human milk will be used to monitor the effects of diet and stage of lactation on the process of milk fat synthesis in the lactating human breast.